1. Field of the Invention
This invention pertains to a demodulator and more particularly to a demodulator which is adaptable for demodulating any signal which has been modulated in amplitude and/or phase by a grouping of conventional or arbritrary signal points.
2. Description of the Prior Art
Increasingly data in the form of a digital signal is transmitted by using it to effect modulation of a carrier signal. The particular type of modulation may follow anyone of a number of well known and conventional modulation schemes, such as phase shift keying (PSK) or quadrature amplitude modulation (QAM). All of these conventional schemes may be considered to be within the class of modulation schemes referred to as amplitude and phase keyed (APK). In addition there are a multiplicity of unconventional APK modulation schemes. Common to all such APK modulation schemes is the generation and transmission at the modulator of an analog signal whose amplitude and phase at any instant is uniquely representative of a predetermined number of data bits. The analog signal has an unique number of such amplitudes and phases which number is dependent solely on the particular modulation scheme being used. A further discussion of various APK modulation schemes may be obtained by referring to the following papers (1) "A Comparison of Modulation Techniques for Digital Radio", John D. Oetting, IEEE Transactions on Communications, Vol. COM-27, No. 12, December 1979 and (2) " Carrier and Bit Synchronization in Data Communication--A Tutorial Review", L. E. Franks, IEEE Transactions on Communication, Vol. COM-28, No. 8, August 1980.
Today such APK modulated signals find their principal application in microwave carrier systems. Such systems are associated with digital radio, land lines and cable networks. As is well known in the art such systems include modems at each end station wherein information to be transmitted is encoded in a modulator in accordance with the selected APK modulation scheme and wherein information received is decoded in a demodulator in accordance with the selected scheme.
With the large number of conventional and unconventional APK modulation schemes it is desirable to have both a modulator and a demodulator which are easily adaptable for use with any one of the APK schemes that may be selected. There have been some attempts in the prior art to design such a demodulator. One such attempt is described in U.S. Pat. No. 3,970,946 which issued on July 20, 1976. The demodulator described therein uses for the purposes of demodulation a reference map which contains the unique number of amplitude and phase points associated with the particular APK modulation scheme being used.
One embodiment of a demodulator useful for demodulating 16-QAM modulation is shown in FIG. 4 of the patent. An embodiment of a demodulator useful for demodulating another kind of modulated signal is shown in FIG. 9. Each of the embodiments differ from each other and make extensive use of hardware in the form of logic circuits. In addition, each of the embodiments includes a control source which is used to generate signals for controlling operation of each demodulator in a manner such that the process of demodulation is carried out in a sequential manner. Finally, while the patent shows in FIG. 8 that read only memories (ROM's) may be substituted for some of the logic circuitry in the demodulator of FIG. 4 it is clear that the control source remains. Therefore, the demodulation process remains unaltered.
Another such attempt is that described in the paper entitled "Carrier Recovery Systems for Arbritrarily Mapped APK Signals" by Matsuo and Namiki and published in the IEEE Transactions on Communications, Vol. COM-30, No. 10, October 1982. The paper describes an arbitrary APK demodulator. A block diagram of the demodulator is shown in FIG. 2 and the essential carrier control circuit for use therein is shown in FIG. 3. The demodulator includes a coarse carrier recovery loop which is used when the demodulator operates in the capture or acquisition mode to recover the carrier associated with the transmitted signal. In that mode the demodulator uses a sub-constellation concept implemented in the form of hardware to control carrier recovery.
The demodulator includes a fine carrier recovery loop which is used when the demodulator operates in the steady state or locked mode after it has recovered the carrier. In that mode the demodulator uses a ROM look-up table to provide control for a feedback loop. Therefore the demodulator described in the above paper uses, dependent on its mode of operation, two separate and distinct carrier recovery loops. In addition as shown in Table I only certain of the control circuits are used in each loop. Thus, the demodulator described therein uses different circuit means and controls dependent on its mode of operation.
It is, however, desirable that a demodulator use the same circuit means and controls independent of its mode of operation. It is further desirable that such a demodulator be easily adaptable for use with any type of APK modulation scheme. It is additionally desirable that such a demodulator include therein a multiplicity of reference signal planes all of which are used in both the acquisition and steady state modes to capture and maintain lock on the carrier signal. It is also desirable that such a demodulator be designed in a manner such that real time, as opposed to sequential, processing be used in both the acquisition and steady state modes of operation. Until the present invention no such demodulator and matched modulator were known.